1. Field of the Invention
The present invention relates to an exposure apparatus used in photolithography process for manufacturing, for example, semiconductor devices and liquid crystal display devices and, in particular, to an alignment system for positioning such members as mask, mask stage, substrate, and substrate stage with respect to each other.
2. Related Background Art
Most of projection exposure apparatuses with sequential exposure method currently in use incorporate therein an alignment system for optically positioning a reticle, as a mask, and a photosensitive substrate (wafer, glass plate, or the like on which a photosensitive material such as resist is coated) with respect to each other. In such an alignment system, various techniques are used. Among these various techniques, TTR (through-the-reticle) technique, in which an alignment mark (reticle mark) formed at the periphery of a circuit pattern of the reticle and an alignment mark (wafer mark) formed at each shot area on the wafer are simultaneously detected by way of a projection optical system, has been known as a highly accurate technique.
In the TTR alignment system, the relative positional displacement amount between the reticle mark and the wafer mark is measured by an alignment optical system disposed above the reticle. The position or rotational angle of at least one of the reticle and wafer is finely adjusted so as to maintain this positional displacement amount within a predetermined range. Here, the TTR alignment system includes various types.
For example, in the alignment system disclosed in Japanese Unexamined Patent Publication No. 3-3224, a correction lens for correcting axial chromatic aberration with respect to alignment light is disposed within its projection optical system. Also, a correction optical system for correcting chromatic aberration in magnification with respect to the alignment light is disposed near the reticle. In the alignment system of this publication, monochromatic light such as laser light is used as the alignment light in order to facilitate correction of chromatic aberration.
Also, in the alignment system disclosed in Japanese Unexamined Patent Publication No. 5-160001, a correction optical device (PGC) for correcting axial chromatic aberration and controlling chromatic aberration in magnification with respect to alignment light is disposed between the reticle and the mask.
On the other hand, so-called off-axis type alignment system has been known as an alignment system other than the TTR type. In the off-axis type alignment system, the wafer mark and the reticle mark are respectively observed by a wafer observation system and a reticle observation system, which are independent from each other, without the aid of a projection optical system. The wafer mark and the reticle mark are positioned outside of an effective field of the projection optical system, respectively. Thus, in the off-axis technique, the optical axis of the wafer observation system and the optical axis of the reticle observation system are widely distanced from the optical axis of the projection optical system. In other words, observation fields of the wafer observation system and reticle observation system are widely distanced from the exposure area on the wafer and the transfer target area on the reticle, respectively.
As more integration in semiconductor devices is demanded every year, smaller pattern rule (line width) has been required for the circuit pattern. The line width which can be resolved by a projection optical system has been known to become smaller in proportion to wavelength. Accordingly, in order to enable exposure of a circuit pattern with a smaller pattern rule, the wavelength of light used for the exposure is shortened. Recently, there have been proposed projection exposure apparatuses using excimer laser light derived from ArF (with a wavelength of 193 nm) and excimer laser light derived from KrF (with a wavelength of 249 nm), for example.